KABUL, Afghanistan — In the first official report of partial results from the Afghan presidential election, the candidates Abdullah Abdullah and Ashraf Ghani appeared to be leading a race in which a runoff election was increasingly certain, according to data released by the Independent Election Commission on Sunday.

The commission warned that these early results, accounting for 10 percent of the votes cast in 26 of Afghanistan’s 34 provinces, could well change as tabulating continued over the coming weeks. The votes will be fully counted by April 24, and a runoff election would be held no sooner than May 28, officials have said.

The results could well be affected, possibly significantly, by widespread fraud at the polls. The election complaints commission said on Sunday that it had received so many serious fraud complaints that it might have to extend the time needed to adjudicate them. The commission said 870 instances of fraud had been classified as serious enough to affect the outcome of the April 5 election, more than the 815 recorded in 2009.

The early results prompted an outcry from the six candidates who did not rank among the top two. After meeting privately to discuss the figures, they issued a joint statement rejecting the partial results as premature, and described the election commission’s decision to release them as an “unforgivable crime.”

Later, the campaign of Zalmay Rassoul, a former foreign minister who was believed to have the quiet support of President Hamid Karzai, issued a separate, and far milder, statement calling on the commission to “transparently distinguish valid votes from fraudulent ballots and not compromise the actual votes of the Afghan people.”

The commission, which had earlier said complaints were fewer than in the hotly disputed 2009 election, apparently reversed that view on Sunday. However, a spokesman for the commission, Nader Mohseni, said the commission could not be sure of any comparisons made between this election and the 2009 vote.

Video

On the Minute, the chief of The Times’s Kabul Bureau, Rod Nordland, discusses the first results from Afghanistan’s presidential elections.Published OnApril 14, 2014CreditImage by Bryan Denton for The New York Times

With about half a million votes counted, Mr. Abdullah was leading with 212,312, or 41.9 percent of the total, followed by Mr. Ghani with 190,561, or 37.6 percent. Mr. Rassoul had 9.8 percent, and Abdul Rab Rassoul Sayyaf, a warlord and former member of Parliament, had 5.1 percent.

Ahmad Yousuf Nuristani, the head of the election commission, warned that “these results are changeable — it is possible that one candidate is the front-runner in today’s announcement but the next news conference may be another candidate as the front-runner.”

In addition, some votes may be disallowed. “We are investigating fraudulent votes very carefully, and there’s a strong possibility that some of the vote will be disqualified,” Mr. Nuristani said.

Even before the results were in, the apparent losing candidates were negotiating with Mr. Abdullah in an effort to form coalitions in a runoff.

“These days everybody is talking to everybody,” Mr. Abdullah said in an interview on Saturday while awaiting the release of the results. “We have no doubt in our mind that by taking care of some of the problems that occurred last time and preventing them from happening again, there will be even a much, much clearer lead and victory, if it goes to the second round.”

Mr. Ghani’s and Mr. Abdullah’s campaigns each had confidently predicted that it would win at least 50 percent of the vote in the first round.

Mr. Ghani, a technocrat and a member of Afghanistan’s largest ethnic group, the Pashtuns, seemed to garner widespread support among them. Mr. Rassoul, also a Pashtun, fared less well, but had strong support among government officials and urban elites.

Image

Ashraf Ghani speaking on Sunday. Mr. Ghani and Mr. Abdullah were well in front with about 10 percent of the April 5 presidential vote counted, but complaints of fraud — more widespread than indicated earlier — could affect the outcome.CreditBryan Denton for The New York Times

Mr. Abdullah, who is half Pashtun and half Tajik, is more closely identified with the Tajiks, who dominate in the north but are less numerous than the Pashtuns. So a Rassoul-Abdullah runoff alliance, which was among those being discussed, could be potent if the two campaigns can negotiate one.

After the results were announced, Mr. Abdullah seemed in no mood to start celebrating, despite being the front-runner. “It’s the beginning of the counting process,” he said at his house and campaign headquarters. “Whether the remaining part of the process works in a transparent and fair manner remains to be seen.”

Mr. Abdullah and President Karzai met on Thursday to discuss the election. “He said that whatever the outcome, the winner will be congratulated by him,” Mr. Abdullah said.

All three leading candidates, among a field of eight, complained about fraud at the polls this year. In 2010 parliamentary elections and the 2009 presidential race, more than a fifth of all votes cast were invalidated because of tampering and other irregularities.

Some spectacular examples of irregularities were noted. In Khost, a member of Parliament forced his way into a polling place at gunpoint and made off with the ballot boxes, according to the complaints commission. One candidate showed a reporter five books of 100 presidential ballots each, all marked for Mr. Ghani, along with a stamp used to authenticate the ballots and voter registration cards enabling people to cast their votes. The candidate, who did not want to be identified because the ballots should have been turned in to the authorities, said he had gotten them from concerned soldiers who had confiscated them before they could be cast.

With many new controls introduced to prevent fraud, it seemed most likely Afghan authorities would be able to counteract a great many attempts at fraud, as they had done in previous elections even without the same high-tech measures in place.

In other ways, though, the election seemed to be unfolding well so far.

Election officials said turnout was expected to have topped seven million voters, and could end up around 7.5 million. Even if a million of those votes were disallowed because of fraud and ballot tampering in some areas, the showing would still be more impressive than the 2009 election, which returned Mr. Karzai to power.

“This is a great historic moment in Afghanistan,” said Mr. Nuristani, the election commission head. “Through this election process, power will be transmitted peacefully from one president to another. It’s a great chapter in our history, and the high turnout, even though there were many threats from the enemies of their country, shows that Afghans want to determine the political destiny of their country.”

Azam Ahmed contributed reporting.

A version of this article appears in print on , on Page A4 of the New York edition with the headline: Partial Afghan Results Portend a Runoff as Fraud Is Cited. Order Reprints | Today’s Paper | Subscribe